1. Field of the Invention
The present invention relates to laminated coil components and methods of manufacturing the same. In particular, the present invention relates to a unique arrangement of coil conductors inside ceramic laminates.
2. Description of the Related Art
A longitudinally laminated and laterally coiled chip inductor disclosed in Japanese Unexamined Patent Application Publication No. 2002-252117 is an example of a laminated coil component. As shown in FIG. 11, a chip inductor 31 includes a coil conductor 33 having an axis that is perpendicular to the laminated direction (thickness direction) X inside an approximately rectangular ceramic laminate 32. The coil conductor 33 having the axis extending in the longitudinal direction Y of the ceramic laminate 32 is provided inside the ceramic laminate 32. Strip electrodes 34 are disposed at the upper portion and the lower portion inside the ceramic laminate 32. The ends of the strip electrodes 34 are connected with each other inside the ceramic laminate 32 through via-holes 35 passing through the ceramic laminate 32 in the thickness direction X to form the coil conductor 33.
The via-holes 35 are provided by forming through-holes at predetermined locations in each ceramic green sheet defining the ceramic laminate 32 and by filling the through-holes with a conductive material (conductive paste), such as an Ag paste. An example of the ceramic green sheet is a ferrite sheet. The two endmost strip electrodes 34 disposed at the upper portion of the ceramic laminate 32 extend to the side surfaces in the longitudinal direction Y of the ceramic laminate 32, and are connected to external electrodes 37 coated on the side surfaces of the ceramic laminate 32, respectively.
Regarding the preparation (not shown) of the ceramic laminate 32 of the chip inductor 31, a plurality of ceramic green sheets having only via-holes 35 are stacked in the laminated direction X. Then, a plurality of ceramic green sheets having strip electrodes 34 and via-holes 35 are attached on the top and the bottom of the resulting laminate of ceramic green sheets. Furthermore, a plurality of ceramic green sheets which do not have the strip electrodes 34 and the via-holes 35 are stacked on the top and the bottom of the resulting laminate of ceramic green sheets.
The ceramic laminate 32 is prepared by press-bonding the laminated ceramic green sheets monolithically along the laminated direction X, and by then firing. Then, the external electrodes 37 are formed on the side surfaces of the ceramic laminate 32 by dipping in a conductive paste and subsequent firing. Thus, the chip inductor 31 having the dipped end surfaces is prepared.
The relative inductance (L) of a coil in a laminated coil component will now be investigated. For example, in the known chip inductor 31, the coil conductor 33 has the highest relative inductance (L) when the inner cross-sectional area (inner area) and the outer cross-sectional area (outer area) of the coil conductor 33 are the same as each other. The highest relative inductance (L) is achieved when the laminated coil component is designed to have a ratio of approximately 1:1 regarding these areas.
In the design of the chip inductor 31, some restrictions must be taken into account. The ceramic green sheets stacked for defining outer coatings on the top position and the bottom position in the thickness direction X of the coil conductor 33 disposed inside the ceramic laminate 32 must have an outer-coating thickness that is greater than a predetermined thickness in order to prevent Ag diffusion. Side gaps in the width direction Z of the ceramic laminate 32 must be greater than a minimum gap required in order to prevent the exposure of the strip electrodes 34 and the via-holes 35 to the exterior regardless of distortion during laminating or cutting.
These restrictions are more noticeable as the outside dimension of the chip inductor 31 decreases. As a result, it is highly disadvantageous to design a coil conductor 33 having substantially equal inner and outer areas.
The ceramic laminate 32 of the chip inductor 31 is prepared by press-bonding a large number of ceramic green sheets, and by firing them after cutting. In general, the conductive material in the through-holes defining the via-holes 35 are not readily deformed during the press-bonding as compared to the ceramic green sheets. Therefore, the conductive material functions as posts for resisting the compacting pressure during the press-bonding, and the via-holes 35 receive the compacting pressure.
Consequently, the compacting pressure applied to ceramic regions near the via-holes 35 which are densely arranged is less than that applied to ceramic regions that are spaced from the via-holes 35. Because of low compacting pressure, delamination and insufficient sintering during firing readily occur at the ceramic regions near the via-holes 35. Furthermore, the conductive Ag material for the via-holes 35 is easily diffused, which results in a decrease in insulating resistance between the via-holes 35.